Most of the energy used in the world today is derived from the combustion of carbon and hydrogen containing fuels such as coal, oil and natural gas. In addition to carbon and hydrogen, these fuels contain oxygen, moisture and contaminants such as ash, sulfur, nitrogen compounds, chlorine, mercury and other trace elements. Awareness of the damaging effects of the contaminants released during combustion triggers the enforcement of even more stringent limits on emissions from power plants, refineries and other industrial processes. There is an increased pressure on operators of such plants to achieve near zero emission of contaminants and to reduce carbon dioxide (CO2) emission.
The art teaches various processes and technologies designed in an attempt to reduce the emission of contaminants from combustion gases. For example, baghouses, electrostatic precipitators and wet scrubbers may be employed in some processes to capture particulate matter. Various chemical processing also may be employed to reduce sulfur oxides, hydrogen chloride (HCl) and hydrogen fluoride (HF) emissions. Additionally, combustion modifications and NOx reduction processes may be used to reduce NOx emissions.
Significant progress has been made in the last twenty to thirty-five years and plants today are much cleaner and safer to the environment than in the past. However, there are growing indications that even small concentrations of particulate matter and especially the very fine, less than 2.5 micron size particles (referred to as “PM2.5”), sulfur oxides, acid mist and mercury may be harmful to human health and need to be controlled. Additionally, in the last few years, there has been a growing concern related to the accumulation of CO2, a greenhouse gas, in the atmosphere. The accelerated increase of CO2 concentration in the atmosphere is attributed to the growing use of fuels, such as coal, oil and gas, which release billions of tons of CO2 into the atmosphere every year.
Reduction in CO2 emission can be achieved by improving efficiency of energy utilization, by switching to lower carbon concentration fuels and by using alternative, CO2 neutral, energy sources. However, CO2 emitting fuels will likely continue to be the main source of energy in the foreseeable future. Consequently, a low cost, low energy consuming process for capturing and sequestering CO2 is needed to assist in reversing the trend of global warming.
One such process for capturing and sequestering CO2 is the Chilled Ammonia Process (CAP) developed by Alstom Power. The chilled ammonia capture method developed by Alstom could remove up to or greater than 90% of CO2 from combustion gasses. The Chilled Ammonia Process is described in U.S. Patent Application No. 2008/0072762.
In the Chilled Ammonia Process, ultra cleaning of combustion gas to near zero concentration of residual contaminants followed by the capture of CO2 is provided. The high removal efficiency of residual contaminants is accomplished by direct contact cooling and scrubbing of the gas with cold water. Through this chilling, the temperature of the combustion gas is reduced to 0° C. to 20° C. to achieve maximum condensation and gas cleaning effect. The CO2 is then captured from the cooled and clean flue gas in a CO2 absorber utilizing an ammoniated solution or slurry in the NH3—CO2H2O system. The absorber typically operates at 0° C. to 40° C. depending on the specific applications. Regeneration is accomplished by elevating the pressure and temperature of the CO2 rich solution from the absorber. The CO2 vapor pressure is high and a pressurized CO2 stream, with low concentration of NH3 and water vapor is generated. The high pressure CO2 stream is cooled and washed to recover the ammonia and moisture from the gas.
Accordingly, as indicated above, the first step of the Chilled Ammonia Process is scrubbing and cooling the gas with cold water to reduce the temperature of the gas to 0° C. to 20° C. to achieve maximum condensation and gas cleaning effect. The combustion gas is cooled by passing the gas by Direct Contact Coolers (DCC). The coolers use chilled water to contact and cool the gas. The water is chilled using one or more mechanical refrigeration systems. The chilled water is then fed through the DCC, which reduces the temperature of the combustion gas, washes and scrubs the gas, captures residual contaminants in the gas, and lowers the moisture content of the gas. It should be understood that different variations of the Chilled Ammonia Process are known. As used herein, the Chilled Ammonia Process refers generally to any carbon capture process that includes the steps of cooling a combustion gas and using an ammoniated solution or slurry to remove CO2 from the chilled gas. It is noted that the combustion gas is cooled not only in the DCC, but also in the absorber and water wash sections. Apart from cooling, a significant portion of the CO2 absorption heat is also removed by using the referenced mechanical refrigeration system depending on the ambient conditions and specific applications.
It is further noted that the mechanical refrigeration systems employed in the afore-referenced CAP system for reducing CO2 in combustion gases and which are used to cool the combustion gases require a significant amount of power. The necessary consumption of power materially decreases the efficiency of the power plant and increases the overall per unit cost of electricity produced by the power plant.
Regarding the afore-referenced mechanical refrigeration systems of the Chilled Ammonia Process, combustion gases are cooled with water chilled by one or more of these refrigeration systems. The water is chilled using vapor compression-direct expansion refrigeration cycles. With reference to FIG. 1 and as also shown in U.S. Patent Application No. 2011/0173981, a known vapor compression-direct expansion refrigeration cycle 123 is shown. The system includes a conduit 124 providing fluid communication between four elements: a compressor 122, a condenser 126, a throttle 128, and an evaporator 130. A refrigerant, i.e., working fluid, circulates in the conduit 124. The working fluid enters the compressor 122 as a vapor. The compressor 122 consumes power to isentropically compress the vapor. The working fluid exits the compressor 122 as a high pressure vapor and flows to the condenser 126. In the condenser 126, heat is rejected from the working fluid at constant pressure. The working fluid exits the condenser as saturated liquid. Next, the working fluid passes through the expansion valve 128 (also called throttle valve). The expansion valve 128 abruptly decreases the pressure of the working fluid at the required evaporation conditions, causing flash evaporation and auto-refrigeration. The liquid then travels through the evaporator 130 and is completely vaporized and sent to the compressor inlet, such that the vapor is directly sent to the compressor inlet. The vaporization absorbs surrounding energy, thereby providing the required cooling effect. The absorption of energy during this step is used to chill the water or process solutions for cooling the combustion gases. The resulting working fluid returns to the compressor 122, thereby completing the refrigeration cycle 123. The refrigeration cycle 123, and specifically the compressor, consumes significant power, thereby reducing the overall efficiency of the power plant.
Accordingly, there exists a need for systems and processes for reducing the power required to perform, e.g., the Chilled Ammonia Process for cleaning a combustion gas.